1. Field of Invention
The present invention relates, in general, to semiconductors and, more particularly, to a novel monolithic semiconductor structure having one or more pairs of semiconductor devices combined on a semiconductor substrate.
2. Brief Description of the Prior Art
As the operating voltage of microprocessors approaches the one-volt mark, operating currents continue to increase. Today's high-end notebook computers consume 20 A. Servers and high-end desktop computers presently require 60 to 90 A. The next-generation GHz class of microprocessors will require current as high as 130 A. These changes in operating conditions challenge power components to maintain and monitor acceptable efficiency levels.
Power components may use isolated forward converters to maintain acceptable efficiency levels and reduce power dissipation. A common isolated forward converter topology for the computer, telecom and networking industry has one discrete vertical planar or trench MOSFET on the primary side and two discrete vertical planar or trench MOSFETs on the secondary side as synchronous rectifiers.
FIG. 1A-B depict aspects of prior art synchronous rectifier circuits. Specifically, FIG. 1A is a schematic of a prior art self-driven synchronous rectifier using three discrete trench MOSFETs, M1, M2 and M3. Alternatively, FIG. 1B is a schematic diagram of an external-driven synchronous rectifier using three discrete trench MOSFETs, M1, M2 and M3. In both topologies, M1 functions as a primary-side control switch, M2 functions as a secondary-side synchronous forward and M3 functions as a secondary-side synchronous catch.
International Rectifier's 30V-rated IRF7822 or IRF6603 (DirectFETTM) products are examples of widely used devices using discrete vertical planar or trench MOSFETs in rectification applications.
Power components may also use two discrete vertical planar or trench MOSFETs to monitor operating conditions—one vertical planar or trench MOSFET functions as a main switch with its current monitored by a second (“external”) vertical planar or trench MOSFET which functions as a sense. FIG. 2 is a schematic diagram illustrating the foregoing prior art configuration. As shown, the source and gate terminals of the power MOSFET switch are connected to the corresponding source and gate terminals of the external MOSFET sense. The drain terminal of the external MOSFET sense monitors the voltage or current at the drain of the MOSFET switch.
Both low device on-resistance and low gate charge are necessary to run high-frequency forward converters or increase power density in the same form-factor. Vertical planar or trench MOSFETs exhibit very low on-resistance but have a high gate charge due to the inherent vertical device structures.
Moreover to greatly reduce parts count, PCB space, and interconnect parasitics, it is desirable to combine secondary side MOSFETs or switch and sense MOSFETs on a single substrate However, because vertical planar and trench MOSFETs have a common backside drain terminal, it is very complicated and expensive to combine two vertical planar or trench MOSFETs on a single semiconductor substrate.
On the other hand, lateral power MOSFETs, that until now are exclusively used in power ICs and as discrete RF devices, offer very low gate charge and reasonably low on-resistance. However, the use of lateral power MOSFETs is limited to small chip sizes and current ratings due to high metal interconnect parasitic resistance.